UMA for measuring how often 2 or more tabs script the same background contents.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index f74d7ca1513b344cccffa260130142f2121bdc76..889867b219cead1e6a44c6cafe5d441bf97f79c6 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -4928,6 +4928,16 @@ void WebContentsImpl::SetFocusedFrame(FrameTreeNode* node,
   frame_tree_.SetFocusedFrame(node, source);
 }
 
+void WebContentsImpl::DidChangeOpener(FrameTreeNode* node,
+                                      FrameTreeNode* opener) {
+  if (!node->IsMainFrame())
+    return;
+
+  DCHECK_EQ(GetOpener(), opener->current_frame_host());
+  for (auto& observer : observers_)
+    observer.DidChangeOpener();
+}
+
 void WebContentsImpl::OnFocusedElementChangedInFrame(
     RenderFrameHostImpl* frame,
     const gfx::Rect& bounds_in_root_view) {